This invention relates to a method of making a rust preventive peripheral joint between two metal panels by hemming working with application of a rust preventive sealant.
In the manufacture of automobile body components such as doors, hoods and trunk lids it is usual to make a peripheral joint between outer and inner panels of sheet steel by hemming the outer panel along an edge thereof so as to hem in a marginal part of the inner panel, and it is also usual to apply a rust preventive sealant so as to fill in the gaps in the joint with the sealant. Ordinarily the rust preventive sealant is applied to the outer panel so as to form a series of beads of the sealant along and at a short distance from the panel edge before placing the inner panel in the jointing position. At the hemming stage the precedingly applied sealant is forced to spread over the lapped marginal areas of the outer and inner panels. After completing the jointing operation the assembly of the outer and inner panels is undercoated by an electrodeposition coating process including a pretreatment, and then intermediate coating and finish coating are accomplished successively.
The rust preventive sealant is used primarily for affording corrosion resistance to the mating panel surfaces in the hemmed joint in compensation for insufficiency of the protective effect of the electrodeposition undercoating treatment in the joint, and also for augmenting physical strength of the joint. To fully accomplish such purposes it is desired that the gaps between the panel surfaces in the joint be completely filled with the sealant. In practice, however, it is not easy to meet such desire and there are various problems relating to the application of the sealant.
First, outflow of the sealant from the joint offers problems. It will be understood that to completely fill in the gaps in the joint with the sealant the quantity of the sealant initially applied to the outer panel needs to be somewhat in excess of a just sufficient quantity. Then it is inevitable that at the hemming operation a portion of the sealant is pressed out of the joint, and considerable man-hours are required for wiping off the extruded sealant. Although the amount of extrusion of the sealant can be restricted to some extent by controlling the position of the initial application of the sealant and the quantity of the applied sealant, it is impossible to surely prevent extrusion of the sealant along the entire length of the peripheral joint in an intricately shaped article such as an automobile door. Furthermore, even if the sealant extruded at the hemming stage is wiped off there occurs further outflow of the sealant from the joint at the subsequent stage of curing the electrodeposited coating by baking because of swelling of the heated sealant. Additional and considerable man-hours are required for removal of the sealant extruded at this stage.
For the above reasons, currently it is customary to decrease the quantity of the rust preventive sealant so as to be slightly insufficient for filling in the gaps in the joint formed by hemming. To prevent rusting or corrosion of the panels in the unfilled region of the joint, a coating sealant is applied to the exterior of the folded edge part of the outer panel along the entire length of the peripheral joint after the aforementioned electrodeposition undercoating treatment. However, the application of the coating sealant is not easy and involves various problems. The application of the coating sealant along the entire length of the peripheral joint needs to be completed within a short tact time of the sealant even though the panel assembly has an intricate shape as in the case of an automobile door, so that there is a possibility of locally mislocating the sealant. Even though the joint is completely sealed with the externally coated sealant, corrosion resistance of the unfilled region of the joint becomes questionable during long use of the panel assembly because the moisture permeability of the externally coated sealant is not strictly zero. Furthermore, it is preferable to seal the joint precedent to the pretreatment for the electrodeposition undercoating because otherwise the pretreatment liquid will remain in the space in the joint and will spout from the joint at the stage of bake curing of the undercoat with an adverse effect on the corrosion resistance of the panel surface in some areas near the joint. As a further problem, in some cases it is impractical or very difficult to apply the external coating sealant after the hemming operation. For example, in automobile doors the application of the coating sealant to a hemmed joint in the back of a door hinge is obstructed by the hinge.